This invention relates to the preparation of high temperature superconducting oxides, and, more particularly, to a process for oxidizing a precursor compound to the correct oxygen content for achieving superconductivity.
One of the most important scientific advancements of recent years has been the discovery of materials that exhibit superconductivity at relatively high temperatures. In the superconducting state, a material has no electrical resistivity, and excludes magnetic flux lines. These characteristics can be used to great advantage in a variety of electrical and other types of devices. Until the recent discoveries, the maximum temperature at which superconductivity was observed (the "critical temperature" or T.sub.c of the material) was about 32 K, which restricted the applications of the phenomenon to those wherein the material could be cooled to very low temperatures. The newly discovered types of high temperature nonmetallic superconductors in some cases have achieved the superconducting state above 77 K, the boiling point of liquid nitrogen, and there exists the possibility of finding room temperature superconductors.
An important class of high temperature superconductors is complex oxides. An example of a superconducting oxide is the widely investigated YBa.sub.2 Cu.sub.3 O.sub.7--x, where x is typically about 0.2 or less. As used herein, a "high temperature oxide superconductor" is an oxide material of the form AO.sub.p, having a superconducting critical temperature of the material greater than about 77 K.
The effectiveness of these oxides as superconductors is highly dependent upon their method of preparation. The superconducting oxides were first prepared by mechanically mixing the non-oxide elements, usually presented in the form of compounds such as oxides or carbonates, heating and sintering the mechanical mixture at a sufficiently high temperature to form a precursor material having a phase that is superconducting except for an oxygen deficiency, and then oxidizing the mixture in a second heat treatment. The mechanical mixing approach, also termed the solid-state reaction method, is somewhat ineffective in achieving a complete mixture and may result in the formation of extraneous phases, with the result that the final oxide may contain non-superconducting regions and have a superconducting critical temperature below that otherwise expected.
In another approach, termed the crystallization method, the non-oxide components are provided in the form of molten nitrates, and co-crystallized to form a homogeneous mixture. The crystallized material is decomposed to the oxide and sintered to form the precursor material, which is then oxidized as for the solid-state reaction method. The crystallization method produces a more intimate mix of the components during the crystallization step than does mechanical mixing, with the result that sintering and oxidation do not lead to as high a content of extraneous phases as in the solid-state reaction method.
With either method for preparing the precursor material, the final step is oxidation. During sintering, oxygen is lost to the atmosphere, and the sintered precursor material does not exhibit the desired superconductivity. The oxidation step is therefore necessary to raise the oxygen content to the correct value, so that the final material has the required oxidation states and stoichiometry. The oxidation treatment is conducted by placing the precursor material into a furnace operating at a temperature sufficiently high to attain oxidation but not so high as to cause formation of undesirable phases. In the case of YBa.sub.2 Cu.sub.3 O.sub.7--x, for example, the sintering temperature is about 900 C. and the oxidation temperature is about 500 C. The furnace atmosphere during oxidation is an oxygen-containing gas such as oxygen, air, or an oxygen/argon mixture. Oxidation has also been accomplished at lower temperatures using an oxygen plasma.
Although oxidation at 500 C is effective, the oxidation step is slow. Many potential applications of oxide superconductors are related to electronic devices, and the devices may not be heated to high temperature or exposed to a plasma in the intermediate processing steps without damage to the previously placed electronic components. There therefore exists a need for another approach to oxidizing the precursor material, that does not require high temperatures or creation of a plasma. The present invention fulfills this need, and further provides related advantages.